In many vehicular applications it is desirable to be able to effectively control the power applied to a drive train, so as to avoid slip and slide of the vehicle's wheels. This is particularly the case for rail based vehicles, such as locomotives, where it is important that the vehicle be able to accelerate up to operating speed in minimal time and to decelerate rapidly to a stop condition.
Locomotives and other rail based vehicles obtain traction and braking forces by means of friction between the wheel surface and the rail surface. Typically, slip and slide occur when the forces applied to the wheels of the locomotive are larger than the maximum friction force between the wheel and rail surfaces. Slip occurs when forward torque is applied to the wheels. Slide occurs when a braking force in the form of reverse torque is applied to the wheels.
In a conventional locomotive, power control is provided by way of a throttle manually set by a locomotive operator. The throttle command is converted into a corresponding electric current supplied to a plurality of electric traction motors whose rotors are drivingly coupled through a pinion gear to axle and wheel sets that are rotary mounted on the vehicle. Under normal operating conditions, the propulsion system is so controlled as to establish a balanced state at each discrete throttle position as set by a driver of the locomotive. In diesel locomotives, an alternator and rectifier bridge supplies a DC current to a plurality of traction inverters providing a controlled alternating current (AC) to a plurality of electric traction motors.
Maximum traction is achieved when each powered wheel of the vehicle is rotating at such an angular velocity that its actual peripheral speed is marginally higher than the actual speed of the vehicle. The difference between the actual velocity of the vehicle and the wheel speed is generally referred to in literature as creep. There is a low margin in the value at which optimum tractive effort is realized, known as optimum creep.
As persons skilled in the art will appreciated, finding the right throttle setting to achieve optimum creep can be difficult to achieve. In many instances the locomotive operator will apply too much throttle resulting in wheel slip and have to subsequently ease off the throttle to maintain fraction. Conversely, during braking, too much braking force can be instructed by the locomotive controller, resulting in wheel slide.
It would be advantageous if there was provided a system which improved the adhesion characteristics of a wheeled vehicle.